Picture receiving system



Aug. 17,

1943. G. R 'CLARK PICTURE RECEIVING SYSTEM Filed Jan. 1v, 1941 2 sheets-sheet 2 CHEN/VEL A (Tl/15H61);

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ZERO ,B/HS LIM! TIA/G Porz/WML aim-565 w//Lw 61 M' curan' Porn/wml, ackossd wlw/V61 I6 m' z mo Blas Zmventor `Gilbert ARCLar' Patented Aug. i7, i943.

Ir- G sYs'raM Gilbert R. Clark, Staten Island, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 17, 1941, Serial No. 374,826

` (ci. 11s- 7.3)

12 Claims.

of signal channels over which trains of signal` energy representing the same optical image have been transmitted.

In picture transmission systems wherein subcarrier frequency modulation is used, it is known that noiseaifects a received picture to an objectionable degree only when the noise peaks are equal to or greater than 1.4 times the RMS signal. Even for the smoothest noise, which is not necessarily common on a radio transmission circuit, the noise peaks are 3.4 times the RMS noise level. The phase variations that the average noise causes on the signal has a negligible effect compared to the noise peaks which when stronger than the signal actually replace it. This is especially true when the frequency shift range covers most of the pass band used at the receiving station. It is therefore desirable that a two-channel system depending upon the second harmonic of the signal when the fundamental fades below the noise should remain on the fundamental channel until the peak signal in this fundamental channel becomes less than .the noise peaks therein.

Providing the general signal to noise ratio on the received signal is high and provided that there is selective fading of the signal there will then usually be a useable second harmonic frequency at such times that the fundamental frequency signal fades below the noise level. If the second harmonic signal is actually sent from the transmitter then there will be a useable second harmonic almost every time the fundamental signal fades at only a slight reduction in the strength of the radiated fundamental signal.

According to this invention there is provided an electronic switch which is controlled by the amplitude of the noise on the fundamental frequency to select the `harmonic or auxiliary frequency when the noise reaches a predetermined peak value.

The principal object of this invention is to provide a facsimile system whose transmission is less affected by noise or selective fading.

Another object of this invention is to provide a circuit adapted to switch from one frequency to another when the first-mentioned frequency is subjected to noise disturbances or fading Other and incidental objects of the invention Figure 1 is a combined circuit and block diagram showing one form of this invention,

Figure 2 is a circuit diagram showing another form of this invention,

Figure 3 is an explanatory diagram relating to the operation of the invention,

Figure 4 is a circuit diagram showing still another form of this invention,

Figures 5, 6 and 'l are graphical illustrations relating to the operation of this invention in one or more of its preferred forms.

Referring .now in more detail to Fig. 1, there y is illustrated a cam I mechanically coupled to .a motor whose speed is controlled by incoming synchronizing pulses and upon whose shaft is mounted a helix which is adapted to print a picture image and has been described more fully in an article entitled Facsimile Transmission and Reception, by Maurice Artzt in the Electrical Engineering Handbook published by John Wiley and Sons, Inc. A movable contact 3 of a switch including a fixed contact 5 is supported adjacent the cam I by an adjustable slide member 'I which may be xed in a desired radial position with respect to the cam I by a set screw S.

A composite facsimile signal including picture signals and synchronizing pulses are applied to a transformer I I from a suitable system, such as the type shown and described in an article entitled Radio Facsimile by Sub-Carrier Frequency Modulation by R. E. Mathes and J. N. Whitaker, on page 131 of the RCA Review for October, 1939, and impressed across a potentiometer I3 whose variable tap I5 is connected to the control electrode Il of the tube I9. The tube I9 includes a cathode 2l and an anode 23. The signal from the anode 23 which includes both the fundamental and second harmonic signal is applied to the primary of transformer 25 and thence to a channel including a channel filter 2l which separates the fundamental frequency signal from the harmonic frequency signal and applies the fundamental frequency signal to a limiter doubler 29. The fundamental frequency signal is also applied directly to the electronic switch 41. A portion of the fundamental frequency signal is also applied to the primary of a transformer 3l through resistance 32 during the short interval of time which the contacts`3A and 5 are caused to come together by the cam l. The adjustable arm 1 of the switch contacts is adjusted so that the cam closes the switch including terminals 3 and 5 during that time occupied by the synchronizing pulses.

It therefore follows that if the synchronizing pulses are of the no signal variety, that is, if the carrier of the picture transmission lsystem is cut off during the time of the synchronizing pulse the signal applied to the transformer 3| will be comprised of the noise which is present on the fundamental carrier frequency.

This noise signal is rectified in tube 33 and is applied to' a storage condenser 35 and resistor 31 which tends to integrate the power of the noise signals to be applied to the control electrode 39 of the tube 4I. The tube 4I includes a cathode 43 in Whose circuit there .is a meter 45 adapted to indicate the current ilow through the tube 4I.

The input signal to the tube I9 is adjusted by moving the variable tap I5 on the potentiometer I3 to a value which causes the meter 45 to indicate a predetermined amount. The signal passing through the tube I9 is then dependent upon the amplitude of the noise on the fundamental frequency channel which as described is measured during the time interval occupied by the synchronizing pulses.

It is preferable that tube I9 is of the variable mu type whereby the amplification of the tube I9 may be automatically controlled'by the po i tential across the resistance 31. This may be accomplished by any of the well known automatic volume control means.

The signal from the limiter doubler 29 is applied to the electronic switch 41. 'I'he second harmonic signal from the channel filter 21 is also ,applied to the electron switch 41, and a control through a limiter 49 to a converter 5I which may be of the type shown and described in the above identified article by Artzt` entitled Facsimile Transmission Reception.

With regard to the operation of the system, the electronic switch can be adjusted when the modulating signal is removed at the transmitter leav ing only the carrier signal. Under these conditions only noise is heard at the receiver, the peak value of which may be adjusted to a certain value, eithermanually by adjusting control I3 so that meter 45 indicates a predetermined amount, or automatically by using the integrated voltage stored in condenser to control the amplification of a. variable mu tube I9.' When the peak noise has been adjusted to acertain known value, the electronic switch may then be so adjusted that it is not affected by the above mentioned noise peaks, but is as close to this point as physically possible. The gamin control gilcfuit of electron switch 41 is then reduced by When the electronic switch is so adjusted, the

signal peaks have to be equal to or greater than the noise peaks in order for the combined peaks to operate the electronic switch. Wheneverthe fundamental frequency signal becomes this' no longer useful and there is -a possibility that' the harmonic signal may be useful.

According to this method of operation, it is necessary to assume that the noise level will remain constant once the adjustments have been 75. which occuryery infrequently, the control level set. 'I'he xed adjustment method depends upon the noise level, remaining constant while the fundamental signal fades up and down. It is desirable that the adjustments be continually monitored throughout the transmission because, with a fixed control, if the noise level rises, the adjustments are incorrect and the noise may show on the picture. If the noise level drops instead, the electronic switch control is then set unnecessarily high and, in the case of the simple control, may cause the switch to change over to a poor harmonic signal while the fundamental signal is still satisfactory. An oscilloscope monitor may be provided in order to monitor the signal.

A more preferable method of operation is to interrupt the modulation of the transmitter for the duration of each synchronizing pulse. A switch such as indicated by terminals 3 and 5 on the` receiving machine would then make contact during the synchronizing pulse whereby an auxiliary circuit, as shown coupled by transformer 3I, would cause the noise during the time interval occupied by the synchronizing pulse to l be rectied in rectifier tube 33 and integrated in ured during the course of the picture and will indicate on meter 45 where the proper adjustment of the variable tap I5 on potentiometer 4I3 must be set. l

When the noise peaks are derived from synchronous signal interruptions, it is possible to use them for direct control of the gain of the amplier including the tube I9 in a manner such as used for the automatic volume control of signais wherein it is preferable that the tube I9 should be of the variable mu type. This would eliminate the manual control I3.

The circuit shown in Fig. 1 may be utilized for obtaining the optimum noise level control. Once the switch circuit is adjusted and standardized, all control for the circuit conditions may be made -by the change of the control I5. Condenser 35 charges rapidlythrough tube 33 and discharges through resistance 31. The resistance capacita7 circuit has a comparatively large time constant so that it is, therefore, largely infiuenced by the peak noise pulses. When the value of the control I3 has either been adjusted manually or automatically so that the peak noise pulses, as in the case of manual adjustment, being indicated on meter 45 are of a certain predetermined value, the system is in proper adjustment. In the case of manual adjustment of the control I3, meterv 45 must be conti-nuously monitored throughv the picture. However, this is not necessary when automatic volume control is provided. Since it is the peak noise which generally affects the picture, it is desirable that this part of the noise be used for controlling the electronic switch; however, not only should the absolute value of the peaks be considered, but, in addition, their rate of occurrence should also be considered.

For example, if a signal has high noise peaks may be set so that these peaks are allowed 'to aect the picture such as will produce an occasional dot in the picture. The control levelv should even permit lesser peaks to affect the picture if they are likewise infrequent in occurrence. In other words, the control voltage should depend both upon the peak noise and also partially upon the RMS noise.

Turning now to Fig. 2, wherein like numerals refer to similar parts, the noise pulse signal is applied to the rectifier tube 33 through transformer 3l. The output of the rectifier tube 33 includes the storage condenser 35 and the discharge resistor 31. There is also provided a series resistor 53 in the charging circuit of the condenser 35.

If resistance 53 equals resistance 31 and the resistance of the rectifier 33 is comparatively low, the charging rate of condenser 35 through resistance 53 is equal to the discharge rate of the condenser 35 through resistance 31 and the potential across the condenser is approximately proportional to the RMS noise. Any desired time constant can be used to filter the noise.

If the resistance 53 is omitted such as shown i-n Fig. l, the condenser 35 charges rapidly through the rectifier 33 and discharges slowly through resistance 31. Therefore, the condenser 35 charges to very near the peak signal noise potential.

An intermediate value for resistance 53 is preferable and resistance 53 may be made variable withoutl affecting the discharge time constant, thus permitting the control voltage to depend upon any intermediate degree of RMS noise to peak noise.

The fact that the time constant of condenser 35 and resistor 31 will delay the electronic switch from going over from one channel to another is a definite advantage, since the rate of signal fade may not be too rapid in itself. Furthermore, it has already been iound'that the condenser resistor circuit must be such that it will filter irregular noise peaks to avoid unnecessary chattering of the electron switch between one channel and the other. i

Such a circuit functions due to the fact that the noise is highly peaked and of short duration, while signal variations such as fading are relatively slower. This feature makes it possible to differentiate between the noise and the signal to some extent.

A peak signal, peak noise discriminator for electronic switch control purposes should become effective when the peak noise exceeds the peak signal. With this border line condition, the RMS values of the noise and the signal are tabulated as follows:

Peak RMS Peak signal 1.00 0.71. Peak noise 1.00 Up to i30 (max.) foi-'smooth H0159. Peak signal plus Peak noise. 2. 00 0.7i (min.) up to 0.77 (max.) i'or smooth noise.2

ceeds 2.6 times the RMS signal plus noise, regardless of the actual level of the'signal 'and-the noise due to fading. Y i

The 2.6 gure may be understood from the graph shown in Fig. 3, the upper line being 2.6 times the highest points on the lower line for the border line condition.

Referring to Fig. 4, a transformer 55 is provided with a' center tap secondary 51. One terminal of the secondary is connected tothe anodes 59 and `6i of rectifier tube 53 which also includes a cathode 55. The opposite terminal of the secondary 51 is connectedto the anode 51 .of the rectifier tube '69; The anode 12 of the rectifier tube 69 is connected to the anode 5| of the rectifier tube 63. A potentiometer 1I is connected in series with, the cathode 465. and the center tap of the secondary 51 so that a potential results across the potentiometer 1l when there is a signal applied tothe transformer 55. A resistor 13 is connected in parallel with condenser 15 and this combination is connected in series with a resistor 11 which is connected to the cathode 10 of the rectifier tube 59. It therefore follows that a signal impressed upon a transformer 55 also causes a potentialacross the re' sistors 13 and 11.

The resistor 19 is connected to the variable tap of the potentiometer 1l to supply a variable input voltage to the tube B l whose output circuit includes the rectifier tube 83 which provides a direct current control voltage across the resistance-capacity circuit including the condenser 85 and the potentiometer 81.

The cathode circuit of the tube 8l includes a resistor 89 and a positive potential for the anode of tubel 8l is supplied through resistor Si. There is also provided a resistor 93 connected between the positive voltage supplyand the cathode of the tube 8l.

Ei or the potential resulting across the resistor 13 is part of the RMS signal plus noise voltage as full wave rectified, a voltage rectified by tube 53 and filtered by capacity condenser 15. Condenser 15 charges slowly through resistance 11 and discharges slowly through resistance 13. Hence, the value of Ei is approximately an RMS value. The time constant of the condenser 15 and the resistance 13 must be large enough to smooth out the noise peaks and small enough to permit E1 to follow the fading of the signal. It may be necessary in order to filter the noise peaks to use a larger time constant than that required to pass the most rapid types of fading.

E2 is part of the signal plus peak noise voltage as half wave rectified by tube 53. It will be noticed that no filter is used on this voltage developed across resistor 1l. It is permissible to get the peak voltage by half wave rectification as shown since the signal and noise are obtained l following a band pass filter which provides a signal such as Ishown in Fig. 5b.

The Variable tap on resistance 1l is adjusted so that at the border line condition, that is, when the peak noise equals the peak signal, the peak value of E2 equals the RMS value Ei. When this adjustment is made the variable tap on the resistance 1l may be fixed and should not require any further adjustment.

Since the polarity of El and E2 are different, Ez subtracts from the value of Ei; the signal plus noise peaks of Ez just balance the RMS voltage of E1 regardless of the actual value of the A'signal and noise. When the noise peaks are greater than the signal peaks it automatically follows that the peaks of En will more than balance E1, since the RMS voltage Ei is only slightly increased by large increases in the peak noise.

The voltage divider includingresistances 88 and 93 keeps tube 8| just at cut off voltage when E1 minus E2 is equal to zero. Therefore, it follows that tube 8| does not draw current until the peak of- E2 more than balances Ei. When these positive peaks are applied to the tube 8| their magnitude is limited by resistance 19 to the zero bias-point cf tube 8| caused by the potential drop across the resistor 19 when the control electrode of the tube 8| draws current by reason of control electrode rectification.

This is clearly indicated by the graphical illustration of Fig. 6. When tube 8| is at or beyond cut-olf voltage, condenser 85 charges to a part of the plate supply voltage depending upon the voltage drops in the circuit including resistance 81, tube 83 and resistance 9|. If resistance 9| is large compared to resistance 81, condenser 85 will charge to a certain small part of the plate supply voltage during the long intervals in which tube 8| is at or beyond cut o.

During the momentary intervals in which tube Y 8| is made conductive by noise pulses, the anode potential of tube 8| is lowered by the increase in current through resistance 9|. The charge on condenser 85 is then at a higher potential than the plate of tube 8| and therefore rectifier 83 becomes non-conductive because of the fact that its cathode is at a higher potential than its anodes. Condenser 85 then proceeds to dis'- charge through resistance 81 until its potential drops to the lowered plate potential of tube 8|.

Since resistance81 is small compared with resistance 9| condenser` 85 discharges through resistance |31v faster than it is later recharged through resistance 9|. However, the time constant of condenser 85 and resistance 81 is long enough so as not to permit the potential across condenser 81 to drop to the lowered plate potential of tube 85 in the time of a single noise peak. Condenser 85 can only discharge this much following several successive noise peaks through tube 8|. This is illustrated in Fig. 7. The output obtained across potentiometer 81 may be used to control an electronic switch.

It is not the intention of the applicant that this invention be restricted in its application to picture receiving systems, but'itmay be applied to practically -any type of radio or wireline frequency modulated signals, such as telegraphy and printer signals.

While several systems for carrying this invention into effect have been indicated in the drawings, it will be apparent to one skilled in the art that this invention is by no means limited to 'theselecting one of said trains of composite signals,

and means responsive only duringsaid synchro- ,nizing pulses for controlling said selection to another of said trains when said selected train fades below a predetermined value.

2. A picture receiving system normally responsive to a single train of signals, having means for Vapplying to said system at least two trains of composite signals, each train representative of the same optical image, and each train including picture signals and synchronizing pulses, and wherein said synchronizing pulses are of lesser amplitude than said picture signals,

and means responsive only during said synchronizing pulses for measuring the interfering signal and switching the response of said system to a signal train other than the one to which said system is normally responsive'.

3. lA picture receiving system normally responsive to a single train of signals, means for applying to said system at least two trains of composite signals, each representative of the same optical image, and each train including picture signals and synchronizing pulses, and wherein said synchronizing pulses are of lesser amplitude than' said picture signals, means for selecting one of saidtrains of composite signals, and means for measuring the interfering signal occurring during said synchronizing pulses for switching said system to be responsive .to a signal train other than the train to which said system is normally responsive.

4. A picture receiving system having means for applying to said system at least two trains of composite signals, each representative of the same optical image, and each train including picture applying to said system a composite signal train and a train of signals representative 0f a harmonic of the modulating signal thereof, each representative of the same optical image and each train including picture Asignals and synchronizing pulses, and means responsive to the noise signal only during said synchronizing pulses to measure said noise and to switch said system to said harmonic when said noise reaches a predetermined level with respect to said signal train.

6. A picture receiving system having means for applying to said system a composite signal train and a train of signals representative of a harmonic of a component thereof, each representative of the same optical image, and each train including picture signals and synchronizing pulses, means for selecting one of said trains of composite signals, and means responsive to the signal occurring during said synchronizing pulse period for controlling said selection to said harmonic when said signal train fades below a certain predetermined level.

7 A picture receiving system having means for applying tosaid system the fundamental frequency and at least oneharmonic frequency of asados@ 8. A picture receiving system having means for applying to said system the fundamental frequency and atleast one harmonic frequency of a composite signal train including a picture signal and synchronizing pulses, a selective circuit normally responsive to said fundamental frequency. and means responsive to the amplitude of said fundamental frequency only during said synchronizing pulse tc cause said selective circuit to be responsive only to said harmonic frequency when the signal amplitude during said synchronizing pulse period exceeds in amplitude a predetermined level with respect to said fundamental frequency signal.

- station normally responsive to a single train of signal energy, means for applying to said station a signal train, and a harmonic signal thereof, and means responsive only during interruptions of said fundamental signal train for measuring the amplitude of the interference with respect to said fundamental signal train and causing said station to be responsive to said harmonic signal when the interference to said fundamental sisnal train exceeds a predetermined relative value.

ll. in a. communication system, e. receiving station normally responsive to a fundamental frequency signal, means for applying to said station a signal and a harmonic frequency signal thereof, and means responsive to the interference to said fundamental frequency signal only during predetermined periodically recurring and relatively short time intervals for measuring the amplitude of saidundamental frequency signal relative to said interference and to switch said station to said harmonic signal when said fundamental signal faded to a predetermined relative value.

l2. Ina communication system, a receiving station normally responsive to a single signal train, means for applying to said station a periodically interrupted signal train and a harmonic frequency signal thereof, means responsive to the interference of said signal train during said interruptions for measuring said, interference, and means for switching said station to said harmonic signal when said signal train l faded to a predetermined level.

, GILBERT R. CLARK. 

